We hear from liberals that unlawful voting doesn’t happen, and particularly that illegal aliens don’t cast ballots. But the Public Interest Legal Foundation has investigated voting behavior in Virginia and found a different story, that in fact thousands of illegals voted there unlawfully.

When Maureen Erickson registered to vote in Prince William County, she listed her home address as a street in Guatemala, in what should have been a very strong indication that she wasn’t a regular Virginia resident.

Yet she remained on the voting rolls for years, and even cast ballots in 14 different elections, up through the 2008 presidential contest. She was only purged in 2012, just ahead of the election, after she self-reported as a noncitizen, according to a new report released Tuesday by the Public Interest Legal Foundation.

Ms. Erickson was one of more than 5,500 noncitizens who were registered to vote in Virginia this decade, and were only bumped from the rolls after they admitted to being ineligible. Some 1,852 of them even managed to cast ballots that were likely illegal, though undetected, the PILF, a conservative voter integrity group, said in its report.

Just as troubling, the PILF said, was Virginia’s efforts to try to hide the information from the public — a problem foundation President J. Christian Adams said began at the very top, with Gov. Terry McAuliffe.

“At the instruction of Governor McAuliffe’s political appointees, local election officials spent countless resources to prevent this information from spilling into the open,” Mr. Adams said in a statement releasing the report. “From NoVa to Norfolk and all urban and rural points in between, alien voters are casting ballots with practically no legal consequences in response.”

The report should give an early sense for the problems that are likely to be turned up by President Trump’s new voter integrity commission, which has been charged with getting to the bottom of both the scope of illegal voting and barriers to legal voters casting their ballots.

Edgardo Cortes, Virginia’s elections commissioner, said the new report actually represents a victory, showing how well Virginia does at weeding out voters who admit they are ineligible.

“As the PILF report shows, Virginia election officials have been and are continuing to promptly remove individuals who self-identify as non-citizens from the statewide voter registration list in accordance with federal and state law,” Mr. Cortes said in a statement.

He also said Virginia publishes its efforts to maintain voter lists, including removal of voters who died, were deemed ineligible or moved.

“As part of the McAuliffe administration’s commitment to transparency, this information, and other information cited by PILF, is readily available on our website,” he said. “It is hard to ‘uncover’ something that is readily made available to the public on an ongoing basis.”

The reports Mr. Cortes pointed to do give a number for noncitizens purged from the rolls each year, but do not include the more extensive details — such as the woman who gave her address as Guatemala — that the PILF was seeking.

Mr. Trump singled Virginia out as a locus of massive voter fraud after last year’s election. In late November, explaining his shortfall in the national popular vote, Mr. Trump said there was “serious” fraud in Virginia, New Hampshire and California.

He didn’t give specific evidence, but attributed a lack of attention to the issue to “serious bias” in the press.

“Big problem!” Mr. Trump said on Twitter.

Mr. Cortes at the time said there were no indications of fraud, saying that the ballots cast were counted properly.

But Mr. Trump’s contention is not that valid votes weren’t cast. He says, rather, that millions of ineligible voters are being allowed to vote.

The most ineligible voters counted in any year in Virginia by the PILF were 1,065 votes cast during the 2008 presidential election year.

No academic analysis has documented that large a problem — though voting integrity advocates say that’s partly because nobody is looking very hard.

The new PILF report is meant to put some numbers behind the scope of fraud.

Fairfax County, Virginia’s largest jurisdiction, also notched the most reports of noncitizens who had to be kicked off the rolls, with more than 1,000. Prince William County was second with 523, and Virginia Beach City was third with 517.

The PILF said those numbers could be just a fraction of the problem given that they only cover noncitizens who somehow admitted to state officials that they weren’t legally able to vote.

Cameron Sasnett, the registrar in Fairfax County, said he’s bound by the laws and cannot be proactive in trying to suss out noncitizens on his rolls.

He said he waits for notification from the state, which relies heavily on immigrants who tell the Department of Motor Vehicle they aren’t citizens. He declined to guess at how widespread noncitizen registration might be in Fairfax, saying those were questions he left up to the state.

“We do everything we are required by law to do,” he said.

Noncitizens are not supposed to register, but many of them do anyway because of U.S. laws meant to encourage higher turnout. The National Voter Registration Act, dubbed “Motor Voter,” required states to offer voter registration at places where government officials are likely to encounter citizens, such as motor vehicle bureaus.

Virginia says it’s trying to tap into Homeland Security databases to try to use them to weed out noncitizens, but says the federal government makes it almost impossible to harness them for the purpose of cleaning up voter rolls.

Kansas Secretary of State Kris Kobach, vice chairman of Mr. Trump’s new voter integrity commission, said he expects the panel will use those databases to try to figure out the scope of the problem for at least some states.